The volumetric flow rate and the mass flow rate are defined as the volume and the mass, respectively, of the fluid that passes through the cross section of the flow tube in a unit time. During operation of such a vortex flow sensor, a Kármán vortex street is, as is known, formed downstream of the bluff body, and the pressure fluctuations of this vortex street are converted by a vortex sensor device into an electric signal whose frequency is proportional to volumetric flow rate and fluid flow velocity, respectively.
U.S. Pat. No. 6,003,384 discloses a vortex flow sensor as is commonly used today for measuring the volumetric flow rate or the flow velocity of a fluid flowing through a flow tube having a tube wall, the vortex flow sensor comprising:                a bluff body, disposed along a diameter of the flow tube, which serves to shed Kármán vortices and is connected to the tube wall of the flow tube from inside at a first and a second fixing location which ar located opposite each other; and        a vortex sensor device responsive to pressure fluctuations caused by the vortices which is fitted downstream of the bluff body in a hole formed in the tube wall of the flow tube and seals off this hole, the center of the hole and the center of the first fixing location of the bluff body being located on an element of the cylindrical surface of the flow tube, and said the vortex sensor device including a diaphragm covering the hole and having a first surface facing toward the fluid and a second surface facing away from the fluid, a wedge-shaped sensor vane, attached to the first surface of the diaphragm, which is shorter than the diameter of the flow tube and which has principal surfaces aligned with the element of the cylindrical surface of the flow tube as well as a front edge, and a sensing element attached to the second surface.        
If, in addition, the temperature of the fluid is measured, further properties of the fluid, particularly its current thermodynamic state, can be determined by taking into account the instantaneous density of the fluid and, if necessary, an instantaneous pressure in the fluid, so that a mass flow rate can be measured using the volumetric flow rate. This can be done by means of a microprocessor, for example, which is provided in evaluation electronics that are connected to the vortex flow sensor and process measurement signals provided by the latter.
The above facts were described a long time ago in connection with the temperature measurement in vortex flow sensors with different types of vortex sensor devices. U.S. Pat. Nos. 4,048,854 and 4,404,858, for example, each show a temperature sensor which is so positioned on the inside wall of the flow tube as to be passed over by the flowing fluid.
JP-A 2000-2567 discloses a vortex flow sensor for measuring the mass flow rate, the volumetric flow rate, or th flow velocity of a fluid flowing through a flow tub having a tube wall, the vortex flow sensor comprising:                a vane, fixed at one end to the tube wall from inside by means of a base plate, which in use sheds Kármán vortices, is shorter than a diameter of the flow tube, and has parallel principal surfaces perpendicular to the direction of fluid flow as well as a rounded front surface on which a temperature sensor is disposed;        first sensing elements fixed near the fixing location for sensing pressure fluctuations of the flowing fluid caused by the Kármán vortices; and second sensing elements fixed near the fixing location for sensing deflections of the vane caused by the flowing fluid.        
This temperature sensor, too, is passed over by the flowing fluid and, as the inventors have found, is not resistant to all fluids occurring in operation, i.e., some fluids corrode temperature sensors arranged in this way.
Therefore, these fluids which corrode the temperature sensor must be excluded by the manufacturer of the vortex flow sensors from use together with this temperature sensor. Such an exclusion, however, narrows the range of application of those vortex flow sensors, i.e., the universality of their applications, and hence diminishes their attraction on the market.